For measuring the actual length of fluid cylinders, particularly hydraulic or pneumatic cylinders, there are various technical options available; in particular there are various ways of stroke measurement in a cylinder using sensors, with the sensors being classified as internal sensors and external sensors.
In case of internal sensors the stroke is measured inside the cylinder. Here, for example magnetostrictive sensors, magnetic field sensors, or in case of larger cylinders also cable sensors are used. The advantage of internal sensors is given in that they are very well protected from external influences; however, the expense for integrating them in the cylinders is high. Additionally, the installation is only possible at a certain minimum size, and the length of the completely retracted cylinder (block length) is increased.
Examples for external sensors are cable sensors, laser sensors, magnetic field sensors, or magnetic tapes. Also included in the group of external sensors are here the indirect length measurements, for example by angular or rotary encoders at components moved by the cylinder. These sensors are advantageous in that the construction of the cylinder remains largely unaffected.
Furthermore, it is distinguished between incremental measuring methods and absolute measuring methods:
Incremental length measurements, starting from a reference value, count measuring pulses each of which representing a defined distance. Without any battery buffering, after each loss of voltage, for example when the machine was shut off, the reference position must be assumed anew.
Absolute length sensors are capable of determining the position based on the presently given measurement at any point of time without here being required to assume a reference position if such an assumption of a reference position is or is not possible depends on the respective application and the individual use of the machine).
For example, it is known from prior art to arrange two or more individual sensors about a piston rod. Such sensors, arranged radially in reference to the longitudinal extension of the piston rod, must be connected to each other, on the one hand, and require relatively much space, on the other hand, thus they result in a large design. Furthermore, in case of two (or more) radially arranged individual sensors only the same code position can be detected so that any faulty code, particularly a faulty barcode or a missing barcode, cannot be detected with a certain probability.